This invention relates to the field of scintillating detectors and particularly to the field of detection of light from a scintillating material.
Scintillation detectors are commonly used in down-hole drilling applications, including measurement-while-drilling (MWD), logging-while-drilling (LWD) and wireline logging techniques. In the search for new oil and gas deposits and mapping their extent, a number of measurements are made along the bore holes. Certain instruments placed down the bore hole irradiate the rock surrounding the bore hole and sense radiation emissions from the surrounding irradiated rock.
Radiation from the rock triggers light photon emissions from a scintillation material in the instrument. A photodetector, e.g., photomultiplier or photocell, detects the photons emitted by the scintillation material. Special photomultipliers may be used in the instrument that can withstand temperatures of up to 200° C. encountered several thousand meters underground in a bore hole. By counting the number of photons emitted by the scintillation material, the instrument generates data indicative of the condition of the surrounding rock.
In a conventional instrument, reflectors surround the scintillating material to direct photons to the photomultiplier. Photons can come from any portion or direction of the scintillating material. The photomultiplier does not surround the scintillating material to capture the photons. Traditionally, reflectors surround the scintillating material and provide a mechanism to ensure that all photons from the material are directed to the photomultiplier.
Conventional reflectors often do not direct all photons from the scintillating material to the photomultiplier. Some photons are not effectively reflected by the reflectors and, thus, are not detected by the photomultiplier. Lost photons can seriously degrade the quality of the measurement being made by the instrument. Photons can be lost due to insufficient reflection and loss of reflective properties of the reflectors within the scintillation detector. Further, the reflective properties of the detector may degrade over time due to exposure to a harsh environment (shock, vibration, and thermal expansion) that occurs in an oil-well logging application.
Traditionally, Teflon™ and ceramic reflectors have been mounted around the scintillation material to ensure that light photons are not lost. However, it is particularly difficult to capture photons with these traditional reflectors. For example, the photons emitted through the side or back end of the scintillation material may pass through or be adsorbed by gaps or corners in the reflectors. Moreover, these reflectors have a tendency to become transparent (and thus not reflective) or contain contaminants (which disrupt reflections). Teflon™ and ceramic reflectors tend to become transparent when they are exposed to optical fluids or high compressive pressures. Contaminants in the reflective materials tend to occur when the material is not extremely pure or is exposed to a humid environment, such that moisture becomes trapped in the pores of the reflective material. In view of these difficulties with reflectors, there is a long felt need for a device that reliably and effectively directs photons from a scintillation material to a photodetector.